1. Field of the Invention
The present invention relates to an optical writing device and an image forming apparatus employing the optical writing device.
2. Description of the Related Art
An image forming apparatus, such as a digital photocopier or a laser printer, deflects a laser light to be emitted in accordance with image data in a main scanning direction by rotating a polygon mirror, and forms an electrostatic latent image that is formed on a photosensitive element moving in a sub-scanning direction. An optical writing device in such image forming apparatus generally performs automatic power control (APC). According to the APC control, to keep a light intensity of a laser light emitted from a laser diode (LD) constant, the light intensity of the laser light is detected by a light-receiving element, such as a photo diode, and the value of a current to illuminate the LD is controlled based on the detected light intensity. A photo diode is included in an LD unit together with the LD. More specifically, the photo diode is sometimes arranged in the vicinity of the LD inside the LD unit in some cases.
To eliminate influences caused by, such as individual differences of the LD, time-lapse fluctuations, and temperature fluctuations, the optical writing device that performs the APC control generally performs an initializing operation that determines an initial value of an LD drive current to obtain a target light intensity. For example, according to an initializing operation described in Japanese Patent Application Laid-Open No. 2004-153118, a current supplied to the LD is increased until the light intensity of the laser light emitted from the LD reaches a reference value, and the value of a current at a moment when the light intensity of the laser light reaches the reference value is set as the initial value of the LD drive current.
The image forming apparatus is configured such that the laser light from the LD is reflected by the polygon mirror, and irradiated to the photosensitive element via a plurality of lenses and reflection mirrors. However, when the laser light is reflected by the polygon mirror in accordance with an optical condition of the regular reflection (at 90 degrees of the incident angle), the reflected light sometimes returns directly into the inside of the LD unit in some cases. Such phenomenon is called as a reflected light. If the reflected light is generated, a receiving-light amount received by the photo diode inside the LD unit increases to more than a usual state. For this reason, it is desirable to detect the light intensity of the LD by eliminating influence of the reflected light.
However, according to Japanese Patent Application Laid-Open No. 2004-153118, a current to be supplied to the LD is increased until the light intensity of the laser light emitted from the LD reaches the reference value, and the current to be supplied to the LD is controlled based on the value of the current at a moment when the light intensity of the laser light reaches the reference value, so that the optical writing device tends to receive an adverse effect of a reflected light.
Consequently, sometimes the initializing operation is not properly performed due to an influence of the reflected light, which causes a problem that the quality of an image is degraded due to a failure of initialization.